Wits End With Single Pipe Steam Problem - No Heat in Multiple Risers

Hello all! Having an issue with single pipe steam in my Brooklyn brownstone. Please see photos for the non-performing main, as well as a crude overall diagram. Note that the diagram does not show every fitting/turn/etc. It's just an overview.

My main splits off in three directions from the boiler. Two of the branches work fine. The remaining branch leads to four risers and none of them heat up. The main will be too hot to touch, and the riser, twelve inches from the tee, will be room temperature.

I have read two of Mr. Holohan's books, and they were indispensable in getting as far as I have, but I am now stumped. Do I need to flush the lines? Or is there anything else I can try? Any help is much appreciated!

Background:

I got no tenant complaints last winter. That's not *proof* the risers used to work, but it's suggestive. This year I got complaints from both tenants, within a couple weeks of heating season starting. Things I have tried, in chronological order:

1) I changed the vent on the underperforming main from a C to a Groton #2. This dramatically improved the uptake on the main, but did not help the risers.

2) I insulated most of my mains. This helped mains uptake a little, but did nothing for the risers

3) I added a riser vent (a Groton #2) to one of the risers. This pulled heat maybe 10' up the riser, but *only the first couple of times I cycled it*. I also got mild-to-moderate water hammer on or near that riser for 1-2 days and then that stopped as well.

4) I increased the pitch on the 16' segment (see photos) - it is now about 1" over 16'. This drained a *lot* of water out of what must have been a low spot. Sent my gauge glass from half-full (which is normal) to full. I drained that water out of the boiler.


Other information:

1) The low spot was probably my fault. I replaced a rusted segment of the drain line in the summer, and probably (though I can't be 100% sure) mucked up the pitch in the process.

2) In the off season I (through a sub) added a backup LWC, and a backup Pressuretrol. One interesting thing about the LWC, it never worked. It consistently went into nuisance trip, and after five failed callbacks I bypassed it electrically, and then left it alone. (I never wanted it - it was a local requirement that got spontaneously enforced because of a permit for separate issues. The boiler tech is pissed off at me for cutting them off at five tries and won't come back to remove it).

3) I removed a couple of radiators on the first floor during the last off season as part of a remodel (we got a heat pump for the first floor. I capped their supply lines and left the pre-existing vents in the caps. There were radiators removed from both working risers and non-working risers. All risers' behavior seems unchanged by any of this.

4) My Pressuretrol is set to 2PSI, but the system is controlled by a thermostat - so I thiiiink (?) the Pressuretrol just acts as a safety cutoff.

5) The water in my gauge glass is clear (well, pink from chemical treatment, but not cloudy). The water that came down from the low spot also seems like it was clean. When I drained it, there was some rust in the water, but no more than you'd get opening the drain on any given day.